Cleaning highly heated surfaces such as the outsides of the tubes of water tube boilers and the like has commonly been performed by soot blowers employing steam, air, or a combination of steam and air as the blowing medium. The dislodging of deposits of slag and other fouling materials has become increasingly difficult as the operating temperatures of boilers have increased and the use of fuels of lower quality has become more common.
It has long been known that the use of water, either alone or in combination with a gaseous blowing medium, can increase the ability to dislodge highly adherent deposits, and water has been so used both in soot blowers and in hand lances. It has generally been considered that the effectiveness of water for this purpose is dependent upon a thermal shock effect, which, by tending to shrink and embrittle the deposit, results in fracturing the same so that it will fall or may more easily be dislodged. In order to obtain a sufficient cleaning effect by the use of water in accordance with this previously accepted theory, the danger of overstressing the hot surfaces was substantial, and in fact rapid deterioration of boiler tubes by thermal shock has frequently been caused by such use of water.
The problem is particularly severe in connection with cleaning the water walls in the combustion chambers of large boilers, where the tubes are quite rigidly held in position, and therefore cannot distort in response to temperature induced shrinkage and expansion tendencies as readily as can the pendent tubes. One method of attempting to control thermal shock while utilizing water for boiler cleaning has involved throttling the water supply in such manner that the actual amount of water striking different areas of the tubes will be substantially constant. Where, for example, a water wall surface is to be cleaned by a retracting-type blower which projects a jet in a spiral path against a water wall surface, this former practice would dictate reducing the rate of water flow from the jet, while it is discharing against surfaces closer to the nozzle, to a value lower than that used while it is discharging against surfaces farther from the nozzle, so that the amount of water striking each incremental area of uniform size will be rendered more uniform, the theory being that thermal shock would correspondingly be held within predetermined limits.
Essentially, therefore, prior attempts to utilize water and acqueous solutions to assist in dislodging slag and the like were based upon an unreliable equating of thermal shock with cleaning, from which it often appeared necessary to utilize, or permit, such a high degree of thermal shock, to dislodge the slag, that damaging the tubes was unavoidable. It follows from this that in a great many instances water simply could not be used, because the volume required to dislodge the slag by such methods of operation would damage the tubes. In other instances some damage and shortened tube life have been tolerated in the interests of maintaining streaming rates under difficult cleaning conditions. Disclosures representative of prior thinking are contained in U.S. Pat. Nos, 1,840,545 of Jan. 12, 1932 and 3,344,459 granted Oct. 3, 1967.
By careful field observations and tests, however, I have determined that effective cleaning of slag from boiler surfaces can be achieved while holding thermal shock far below values which would damage the tubes, and that in fact, by practicing my improved method, slag removal can be rendered virtually independent of thermal shock.
The overall objective of the present invention, therefore, is to provide a method and apparatus whereby highly adherent deposits such as slag can be removed from hot surfaces with the aid of water with minimized thermal shock and in a manner which prevents damaging the surfaces.
Other objects and advantages will become apparent upon consideration of the present disclosure in its entirety.